The performance capacity of electrochemical energy storage systems generally diminishes strongly with decreasing operating temperature. This is especially true when dealing with the capability to supply discharge currents which, relative to the capacity of the battery, are one-hour discharge current. In order to counteract this fundamental property of storage batteries various solutions have already been proposed. Thus it is known, for example, that storage batteries may be heated by an external energy source. Use is made of this fact in storage batteries in the military sector which are constantly being recharged in readiness operation. This process is also employed in storage batteries installed in satellites. Furthermore, the above-noted process is used in so-called high-temperature batteries in which the storage battery cells which are combined to form a battery are electrically heated from an external energy source during the charging process. The use of latent heat storage systems to keep storage batteries above a minimal temperature is also known. Further, it has been proposed that the energy stored in the storage batteries be used for heating batteries. This process is utilized especially for high-temperature batteries used for electrotraction, because such batteries are no longer ready to operate below a certain minimal temperature. The temperature of these batteries must therefore always be held above the system-specific minimal temperature, and if necessary the electrical energy stored in the battery is used to increase temperature.
In German patent DE-A 41 42 628, a process and an apparatus are proposed for heating a storage battery in which a power transistor serves as the heating element which is arranged in the battery and is operated by temperature control. The battery is heated as a function of the battery temperature and battery voltage, a lower limiting value being defined for the battery voltage beyond which no heating is performed. In order to avoid unnecessary heating of the battery from its own power supply, a trigger switch is provided. Alternatively, to the proposed trigger circuit which consists of a temperature sensor and a power transistor, a PTC resistor can also be used in combination with a trigger circuit.